The present invention relates generally to current probes and more particularly to a input by-pass circuit for a current probe.
Current probes used with oscilloscopes apply transformer technology to measure current flowing in a conductor. FIG. 1 illustrates a current probe 10 for measuring current signal down to DC having transformer 12 with a ring-shaped magnetic core 14 defining an aperture 16. The ring-shaped magnetic core 14 is preferably a split core design where one side of the magnetic core is movable relative to the other sides. This allows a current carrying conductor 18 to be passed through the aperture 16 of the transformer 12 without having to disconnect the current carrying conductor 18 from a circuit. The current carrying conductor 18 is passed through the aperture in the magnetic core 14 and acts as the primary winding of the transformer 12. A secondary winding 20 is wrapped around one side of the magnetic core 14. To allow the current probe 10 to sense DC and low frequency current signals, a Hall effect device 22 is positioned in the magnetic core 14 such that the magnetic flux in the magnetic core is substantially perpendicular to the Hall effect device 22. A bias source 24 is applied to the Hall device 22 and the resulting voltage generated by the Hall effect due to the flux in the magnetic core 14 is coupled to the input of an differential amplifier 26. The single ended output of the differential amplifier 26 is coupled via a switch 28 to a power amplifier 30. The switch 28 is also coupled to receive a degaussing signal that is selectively applied to the transformer 12. The power amplifier 30 generates a current output proportional to the voltage generated by the Hall effect device 22. The power amplifier 30 is coupled to one side of a secondary winding 20 with the other side of the winding coupled to a transformer termination resistor 32 and voltage amplifier 34.
The current flowing in the current carrying conductor 18 induces a magnetic flux that is linked to the magnetic core 14 and the secondary winding 20. The magnetic flux caused by DC to low frequency current signal is coupled in the magnetic core 14 to the Hall effect device 22. The Hall effect device 22 generates a differential voltage that is coupled to the inputs of the differential amplifier 26. The output of the differential amplifier 26 is coupled through switch 28 to the power amplifier 30. The power amplifier 30 generates a current output that is coupled to the secondary winding 20 of the transformer 12. The current applied by the power amplifier is in a direction that nulls the flux in the magnetic core 14 caused by the DC to low frequency current signal in the current carrying conductor 18. Higher frequency current signals induce a current in the secondary winding that produces a magnetic flux that is opposite to that generated by the current flowing in the current carrying conductor 18. The current generate by the power amplifier 20 for DC to low frequency current signals and the current induced into the secondary winding 20 at higher current frequency signals are summed by the transformer termination resistor 32 which provides a voltage input to the amplifier 34. The voltage output of the amplifier 34 is a measure of the AC and DC components of the magnetic core flux. U.S. Pat. Nos. 3,525,041, 5,477,135 and 5,493,211 describe the concepts of the above current probe 10.
In order for a user to degauss the above described current probe, the current carrying conductor 18 needs to be removed from the current probe 10 other wise the degaussing signal will induce a current into the current carrying conductor 18 that could damage circuitry in the device under test. Additionally, the user needs to remove the current carrying conductor 18 from the current probe 10 if a sufficiently high current is flowing in the current carrying conductor 18 that would saturate the magnetic core 14 of the transformer 12.
What is needed in a current probe is a circuit that allows a user to keep the current probe attached to a current carrying conductor while degaussing the current probe or when a current signal is present that would saturate the magnetic core of the transformer. Additionally, the current probe circuit should allow a user the capability to set an offset voltage for the current probe without removing the current probe from the current carrying conductor.